Electricity from light is one of the most important breakthroughs in modern science. Every time sunlight hits a solar panel, trillions of tiny particles called photons strike the semiconductor surface and free electrons. These electrons then flow as electric current, powering homes, industries, and satellites. This article gives an in-depth explanation of exactly how photons generate electricity, including detailed physics, equations, real-world examples, calculations, and FAQs.
What Is a Photon?
A photon is the fundamental particle of light. It carries electromagnetic energy but has no mass and no electric charge. The energy of a photon depends solely on its wavelength or frequency.
The photon energy is given by the famous quantum equation:
\[ E = \frac{hc}{\lambda} \]
Where:
- \(E\) = photon energy (joules)
- \(h = 6.626 \times 10^{-34}\) J·s (Planck’s constant)
- \(c = 3 \times 10^{8}\) m/s (speed of light)
- \(\lambda\) = photon wavelength (meters)
This equation shows that shorter wavelengths have higher energy photons. For example, blue light has more energetic photons than red light.
Understanding Semiconductor Bandgaps
Solar cells are typically made of silicon, which has two important energy regions:
- Valence Band — where electrons normally stay
- Conduction Band — where electrons are free to move and form electric current
The energy difference between these bands is the bandgap. For silicon:
\[ E_g = 1.12 \text{ eV} \]
A photon must have at least 1.12 eV of energy to excite an electron across this gap. If the photon energy is lower, it cannot generate electricity. If it is higher, the excess energy becomes heat.
How Photons Generate Electricity Step-by-Step
1. Photon Absorption
When sunlight hits the solar cell, photons enter the semiconductor. Only photons with energy \(E \geq E_g\) are absorbed. Others pass through or get reflected.
2. Creation of Electron–Hole Pairs
If a photon has enough energy, it excites an electron from the valence band to the conduction band:
\[ \text{Photon Energy} \rightarrow \text{Electron (conduction)} + \text{Hole (valence)} \]
This creates a negatively charged electron and a positively charged “hole.”
3. Electric Field Separation
Solar cells contain a p-n junction that creates an internal electric field. This field pushes:
- Electrons → toward the n-side
- Holes → toward the p-side
This separation is crucial because it prevents electrons from recombining with holes.
4. Electrical Current Flow
Metal contacts collect the separated charges. Electrons flow through an external circuit, providing electricity.
Electric current (\(I\)) is simply the number of charge carriers passing per second:
\[ I = \frac{Q}{t} \]
Where \(Q\) is charge in coulombs.
Photon Energy Calculation Example
Let’s calculate the energy of a green photon with wavelength \(\lambda = 550\) nm.
\[ E = \frac{hc}{\lambda} \]
Substitute values:
\[ E = \frac{(6.626 \times 10^{-34})(3 \times 10^{8})}{550 \times 10^{-9}} \]
\[ E \approx 3.61 \times 10^{-19} \text{ joules} \]
To convert joules to electronvolts:
\[ 1 \text{ eV} = 1.602 \times 10^{-19} \text{ J} \]
\[ E = \frac{3.61 \times 10^{-19}}{1.602 \times 10^{-19}} \approx 2.25 \text{ eV} \]
This photon (2.25 eV) is well above silicon’s bandgap, so it easily generates electricity.
How Many Photons Are Needed to Produce 1 Watt?
A watt is one joule per second. So the number of photons needed is:
\[ N = \frac{1}{E} \]
Using our photon energy:
\[ N = \frac{1}{3.61 \times 10^{-19}} \approx 2.77 \times 10^{18} \text{ photons/second} \]
That means nearly 3 billion billion photons per second are needed to generate just 1 watt!
Why Solar Panels Are Not 100% Efficient
Several losses occur inside a solar panel:
- Reflection loss — 4–6% of sunlight bounces off.
- Low-energy photons — insufficient energy to cross bandgap.
- Thermalization loss — excess energy becomes heat.
- Recombination loss — electrons and holes recombine.
- Resistance loss — electrical resistance reduces power output.
Modern silicon panels operate around 18–22% efficiency.
Real-World Example: Power in a 350 W Solar Panel
If a solar panel outputs 350 W, and each useful photon carries approximately:
\[ E = 3.61 \times 10^{-19} \text{ J} \]
The number of absorbed photons required per second is:
\[ N = \frac{350}{3.61 \times 10^{-19}} \approx 9.7 \times 10^{20} \text{ photons/second} \]
That is nearly a sextillion photons per second interacting with the panel!
Why Photons Are Ideal for Electricity Generation
Photons are perfect energy carriers because:
- They travel at the speed of light.
- They have no mass and no charge.
- Their energy is predictable from wavelength.
- They interact cleanly with semiconductor bandgaps.
- The sun provides an unlimited supply.
These properties make photovoltaic energy one of the most powerful clean-energy technologies.
Frequently Asked Questions (FAQs)
1. Do all photons produce electricity?
No. Only photons with energy equal to or greater than the semiconductor bandgap can generate electron–hole pairs.
2. Do high-energy photons create more electricity?
No. They still produce only one electron–hole pair. Extra energy becomes heat.
3. Can photons from LED or CFL lights generate electricity?
Yes, but with much lower efficiency because their photon output is weaker compared to sunlight.
4. Is the photoelectric effect the same as photovoltaics?
They are related but not identical. The photoelectric effect ejects electrons from a material, while photovoltaics create a voltage and electric current within a semiconductor.
5. Which wavelength of light is best for solar electricity?
Blue to green photons (400–550 nm) are most effective because their energy matches silicon’s bandgap efficiently.
6. Why can't solar cells use all wavelengths from the sun?
Because the semiconductor bandgap acts as a filter — photons with energy lower than the bandgap cannot excite electrons.
Conclusion
Photons generate electricity by transferring energy to electrons inside a semiconductor. If a photon has enough energy to cross the bandgap, it creates an electron–hole pair that the electric field inside the solar cell separates. These electrons flow through an external circuit and form usable electrical current. With trillions of photons striking every square centimeter of sunlight-exposed surface, solar energy becomes one of the most scalable and sustainable energy sources in the world. Understanding the quantum physics behind photon energy not only deepens scientific knowledge but also supports the development of more efficient and affordable solar technologies.
